One present commercially implemented EAS system has a transmitter which radiates a pulsed magnetic fluid into a surveillance area wherein it is desired to note the presence of articles bearing EAS tags. When a tagged article is present in the surveillance area, its tag is excited by the radiated magnetic field and, based on its composition, is caused to generate a detectable response signal. A receiver, which is enabled between successively spaced transmitter field radiations, detects the response signal of the tag and initiates an alarm or other activity to indicate the presence of the tag in the surveillance area.
A transmitter suited for use in the described EAS system is shown in commonly-assigned U.S. Pat. No. 5,239,696 (the '696 patent), to which incorporating reference is hereby made.
Referring to FIG. 1 of the '696 patent, power amplifier 8 drives resonant antenna 2A and the antenna is driven to provide a recurrent magnetic field substantially unaffected by inductance changes of the antenna brought about such as by objects passing through or present in the surveillance area. Such invariant field conditions are enabled through sensing resistor 9 and summing amplifier 7, the latter providing input to the power amplifier 8.
One facet of the '696 patent of consequence to the subject invention in one embodiment thereof is the envelope shaping effected in circuit 20. The envelope shaping is depicted in FIG. 4 of the '696 patent, i.e., the envelope has adjusted rise and fall times, respectively shown at Tr and Tf. Accordingly, the signal fed to summing amplifier for combination with the sensing resistor 9 signal has a gradual buildup and a gradual falloff. For the purposes of the '696 patent, the envelope wave shaping serves two purposes, namely, providing desired low harmonic content and rapid shutoff of the signal.
Various countries have very strict electromagnetic interference (EMI) limits requiring particularly clean emissions, which, if not met by EAS systems, preclude such countries as a market therefor. Clean emissions can be attained through the use of linear power amplifiers, such as that of the '696 patent. However, pulsed EAS systems involve the production of a high current burst in an antenna for a short period of time. This necessitates that, where a linear power amplifier is used, it must be designed with a significant extra power margin, since linear power amplifiers are typically only about thirty to forty percent efficient. A significant contributor to this inefficiency is the bias current which must flow in all the active devices in the amplifier in order to keep them operating in the linear region of their conduction curves. This bias current causes heat to be generated in the linear circuits, regardless of whether the system is actually transmitting. The linear power amplifier of the '696 patent has such need for continuous bias current.
Another disadvantage attending use of linear power amplifiers in pulsed EAS systems derives from coupling between transmitting and receiving antennas. Many EAS systems use so-called "transceiver" antennas, wherein the transmitter and receiver coils are in very close proximity. In some instances, the transmitting and receiving functions are effected using the same coil. The linear power amplifier is never deenergized. Rather, it simply does not receive an input signal during periods of no transmission. On the other hand, during such no transmission periods, broad spectrum noise is generated in the various energized stages of the linear power amplifier and passed to the transmitting antenna and coupled to the receiving antenna. The noise contains energy at the EAS system operating frequency, and, although of low signal level, the EAS system receiver typically has enough gain that its sensitivity is reduced by this noise. Again, this disadvantage attends the power amplifier of the '696 patent.